Invariant-molecular-dynamics study of the diamond-to-beta-Sn transition in Si under hydrostatic and uniaxial compressions

Abstract

We study the diamond-to-beta-Sn structural phase transition in Si under hydrostatic and uniaxial compressions using an invariant-molecular-dynamics approach based on an empirical potential model. Isobaric molecular-dynamics simulations show that the diamond-cubic lattice under hydrostatic compression becomes unstable against tetragonal shear deformation into the beta-Sn phase at 60 GPa, which is much higher than experimental values. This very high pressure is attributed to the fact that a perfect single crystal is superpressured due to the activation barrier, well above the transition pressure where the two structures coexist, in analogy to isobaric superheating in molecular-dynamics simulations. Under uniaxial compression, the enthalpy barrier for the diamond-to-beta-Sn transition is found to be effectively reduced.